A radio frequency (RF) amplifier is a device used to amplify RF signals in several applications, for example wireless receiver applications. The RF amplifier is coupled between an input circuit, for example a low noise amplifier (LNA), and a mixer to prevent the mixer from loading the LNA.
An exemplary RF amplifier 100, hereinafter referred to as the amplifier 100, is illustrated in FIG. 1 (Prior Art). An output (Vin+ and Vin−) of the LNA is coupled to gates of a transistor 105, a transistor 110, a transistor 115, and a transistor 120 through capacitors, for example a capacitor 125 (Cac), a capacitor 130 (Cac), a capacitor 135 (Cac), and a capacitor 140 (Cac), respectively. In order to prevent the amplifier 100 from loading the LNA and degrading quality factor, the amplifier 100 requires a specific input resistance. Equation (1) illustrates the input resistance of the amplifier 100.
                    Rin        =                  real          ⁡                      [                          1                                                (                                      1                    +                                          gm                      ×                      Zo                                                        )                                ×                s                ×                Cp                                      ]                                              (        1        )            where gm is transconductance of the amplifier 100, Zo is impedance of a load 145, s represents complex frequency and can be determined as s=j*2*pi*f, where f is frequency, and Cp (Cpp+Cpn) is equivalent parasitic capacitance of either the transistor 105 and the transistor 110 or the transistor 115 and the transistor 120.
The parasitic capacitances of the transistor 105 and the transistor 110 are represented as a parasitic capacitor 150 (Cpp) and a parasitic capacitor 155 (Cpn). The parasitic capacitances of the transistor 115 and the transistor 120 are represented as a parasitic capacitor 160 (Cpp) and a parasitic capacitor 165 (Cpn). The parasitic capacitance degrades the input resistance of the amplifier 100 in presence of the load 145. The input resistance of the amplifier 100 also gets degraded when a phase delay from an input (Vin+ and Vin−) to an output (Vo+ and Vo−) deviates by 180 degrees. The degradation in the input resistance loads the LNA and affects gain, frequency selectivity and noise performance of the LNA.
In light of the foregoing discussion, there is a need to prevent loading of the LNA. Further, there is also a need to maintain a common mode voltage associated with the output of the amplifier 100 within a desired range to improve linearity.